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To check for the existence of a particular key in the map, the standard solution is to use the public member function find() of the ordered or the unordered map container, which returns an iterator to the key-value pair if the specified key is found, or iterator to the end of the container if the specified key is not ...
Use the std::map::contains Function to Check if Key Exists in a C++ Map. contains is another built-in function that can be used to find if the key exists in a map . This function returns a boolean value if the element with the given key exists in the object.
You can call map::count(key) with a specific key; it will return how many entries exist for the given key. For maps with unique keys, the result will be either 0 or 1.
find() is used to search for the key-value pair and accepts the “key” in its argument to find it. This function returns the pointer to the element if the element is found, else it returns the pointer pointing to the last position of map i.e “map. end()” . // "it" has address of key value pair.
Use my_map.count( key ); it can only return 0 or 1, which is essentially the Boolean result you want.
Alternately my_map.find( key ) != my_map.end() works too.
Potatoswatter's answer is all right, but I prefer to use find or lower_bound instead. lower_bound is especially useful because the iterator returned can subsequently be used for a hinted insertion, should you wish to insert something with the same key.
map<K, V>::iterator iter(my_map.lower_bound(key));
if (iter == my_map.end() || key < iter->first) { // not found
// ...
my_map.insert(iter, make_pair(key, value)); // hinted insertion
} else {
// ... use iter->second here
}
Your desideratum,map.contains(key), is scheduled for the draft standard C++2a. In 2017 it was implemented by gcc 9.2. It's also in the current clang.
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